In a silver halide photographic element, a color image is formed when the material is exposed to light and then developed using a primary amine color developer. The development results in imagewise reduction of silver halide and the corresponding production of oxidized developer. The oxidized developer then reacts in an imagewise fashion with one or more incorporated dye-forming couplers to form a dye image. At the same time, it is common that at least one of the couplers present in the photographic element be capable of releasing a group capable of affecting the photographic properties of the resulting image. In such cases, a portion of the coupler referred to as the "coupling-off group" is released during the coupling process and this in turn affects the resulting image in a predetermined manner.
Magenta dye-forming couplers are employed in subtractive color forming processes. One of the principal coupler types useful for this purpose are those based on a pyrazolone ring. Pyrazolone-based couplers having a coupling-off group linked to the pyrazolone ring by oxygen have long been considered as potentially attractive two equivalent magenta couplers. An oxygen-linked coupling-off group could impart increased activity to the pyrazolone coupler; however the general instability of these couplers toward ambient oxygen makes them difficult to synthesize and impractical for use in a film environment since they decompose during keeping thereby reducing the density of the dye formed upon exposure and processing. In particular, pyrazolone couplers having an anilino or acylamino substituent at the 3-position have exhibited unacceptable stability when an aryloxy is employed as a coupling-off group. As a result, pyrazolone couplers have employed either so-called "four equivalent" couplers containing hydrogen at the coupling-off position or have employed so-called "two-equivalent" couplers containing a coupling-off group having a sulfur or nitrogen atom linked to the pyrazolone ring. Such couplers have been employed as image couplers where the primary purpose is to form image dye or as image modifying couplers whose primary purpose is to modify the ultimate image to enhance sharpness, granularity, etc.
U.S. Pat. No. 3,419,391 discloses certain types of pyrazolone-based compounds as two-equivalent couplers having high dye-forming reactivity and reduced tendency to form color fog. This is not related to keeping requirements. According to the patent, the pyrazolone ring is not limited to the presence of any particular substituents at the 3-position or elsewhere. Specifically identified substituents at the 3-position include anilino, acylamino, alkyl, amino, alkoxy, amido, carbamoyl, ureido, thio, guanidino, etc. The couplers of the patent may contain an aryl group at the 1-position and, among other things, an alkyl or carboxy ester group at the 3-position. The aryloxy couplers of the patent are said not to produce color fog (printout) and to provide improved reactivity. Thus, they are said to provide low printout or yellowing in Dmin areas when they are exposed to light or high temperatures, respectively, subsequent to development. No mention is made of the poor keeping of pyrazolone couplers having aryloxy coupling-off groups although their instability is well known in the art.
U.S. Pat. No. 4,985,336 discloses a photographic element containing in a green sensitive layer a precursor compound capable of releasing a compound upon reacting with an oxidation product of a developing agent and said released compound is capable of releasing a development inhibitor upon further reacting with another molecule of oxidized developing agent. Several compounds exemplified have a phenoxy coupling-off group with a para nitro and a release group in the ortho position but the presence of a methyl group in the 3-position of the pyrazolone ring does not provide the requisite keeping.
U.S. Pat. No. 5,670,306 discloses a class of pyrazolone-containing compounds as two-equivalent couplers having adequate dye-forming reactivity and good keeping. These compounds, featuring a mono-, di-, or tri-substituted carbon atom at the 3-position of the pyrazolone ring were difficult to prepare, requiring multi-step syntheses.
U.S. Pat. No. 4,686,175 discloses a coupler D-29 which incorporates a 1-aryl-3-aryl-4-aryloxypyrazolone DIR coupler. The inhibitor release mechanism is an intramolecular displacement group the synthesis of which employs phosgene and would be difficult to synthesize.
It would be desirable to have a 1-aryl-3-aryl-4-aryloxypyrazol-5-one based coupler in a photographic element that is stable during synthesis, film manufacture, and during film keeping and that would provide for the effective release of a photographically useful group (PUG) but which would be straightforward to prepare.